Jerry Wilkinson: "Mile Markers (MM) is a relative location of a site in reference to the number of miles north of Key West. The business center of unincorporated Key Largo is at about MM 100; therefore, about 100 miles northeast of Key West. The Florida DOT tries to maintain small rectangular green with white number mile marker signs every mile on both sides of the highway. Generally except Marathon, addresses along the highway is the MM expressed as a four, five or six digit number. For example, a business with an address of 100511 Overseas Highway would located at about MM 100.5. If the extreme right digit (least significant) number is an even number, it is on the west or bayside. If an odd number it is on the east of ocean side. Subtracting two MM's gives the approximate number of miles between two known locations. If the MM's are getting smaller, one is traveling south or southwest. These are listed from north to south. Note: the farther south in the Keys one is, the more east and west the highway runs. Business/commercial location may change or even disappear. This was compiled early 2003. I assume this practice began with Henry Flagler who numbered every mile of railroad beginning at Jacksonville and ending at Key West. They were white and black concrete Mile Post signs along the railroad track and on the printed timetables."
Card Sound Road is a good alternate route to the Keys - greener, quieter, slower. Just before the toll bridge ($1.00), stop at Alabama Jacks (305-248-8741) for refreshment. "Plunked down along Card Sound Road on the outskirts of Key Largo, a lonely stretch of asphalt flanked by mangroves and saltwater, Alabama Jacks looks like a "Gilligan's Island" outpost but holds forth like a country roadhouse." (Where Cloggers and Raccoons Are Regulars by Lizette Alvarez, New York Times, June 18, 2011.) Named one of the seven best dive bars in the Florida Keys (7 best dive bars in the Florida Keys, Barbara Marshall, Palm Beach Post, June 17, 2015.) "Beyond the bridge, on the road through the northern half of Key Largo, I get a glimpse of what the Keys looked like before the highway. It's a rare sight to see....The hardwooods are species with names that evoke their Caribbean origins: sabal palm, humbo limbo, coco plum. Many of thes broad-leaved trees no longer exist elsewhere in the Keys, but here they form dense thickets as sultry as miniature rain forests." (Endless Summer, Bruce Stutz, Conde Nast Traveler, August 2005.)